Cellular wireless communication systems are designed to serve many MS's distributed in a large geographic area by dividing the area into cells. At the center of each cell, a BS is located to serve MS's in the cell. Each cell can be further divided into sectors by using multiple sectorized antennas. Typically three sectors per cell are used. The term sector is used, however, even when there is only one sector per cell. In each cell, a BS serves one or more sectors and communicates with multiple MS's in its cell. The communication between the BS and the MS uses analog modulation (such as analog voice) or digital modulation (such as digital voice or digital packet data) to transmit and receive such data (analog or digital).
In cellular wireless systems, a BS includes devices needed to transmit and receive signals to and from MS's, which typically include modems, up/down converters, ADC's, DAC's, filters, LNA's, power amplifiers, and transmit and receive antennas. A BS also has devices to transmit and receive MS's signals and other control signals to and from other systems such as a base station controller that controls multiple BS's.
A certain amount of bandwidth (radio spectrum) is used for such communication between the BS and the MS. Two separate spectrums can be allocated for the forward link (from the BS to the MS) and for the reverse link (from the MS to the BS) or one spectrum can be time division multiplexed to carry traffic in both directions. The minimum unit of bandwidth needed in a cellular wireless system can be referred to as a carrier. As the amount of data traffic is increased, the number of carriers needs to be increased and/or more BS's need to be installed between existing BS's.
A carrier in a sector can handle up to a certain amount of data traffic, which is referred to as the capacity per carrier per sector or simply capacity. In general, the capacity is different in the forward and in the reverse links.
In a conventional known BS, all functions are implemented in BS. Therefore, the BS becomes a big box to support multiple carriers and sectors, and it occupies a big floor space in the cell site. In another conventional known BS, for a backhaul to a BSC, usually a leased line such as T1/E1 is used. In another conventional known RAN (Radio Access Network) using leased T1/E1 as backhaul, the link between a BSC and a BS is point-to-point STAR configuration. Since a leased line is usually charged per distance, this STAR structure is far more costly than a TREE structure where the total link distance can be greatly saved.
The number of MS's and the amount of data traffic MS's transmit or receive in a cell can vary significantly over time (e.g., heavy traffic during the day in an office area vs. heavy traffic during weekends and evenings in a residential area). It can be costly to have a BS with enough capacity to meet the peak demand. In some cases, the coverage area of a sector does not have enough traffic but still needs the resources of a sector. In this case, it is a waste of a capacity to allocate a sector capacity to a BS.